druid/docs/content/querying/scan-query.md

layout	title
doc_page	Scan query

Scan query

The Scan query returns raw Druid rows in streaming mode. The biggest difference between the Select query and the Scan query is that the Scan query does not retain all the returned rows in memory before they are returned to the client.
The Select query will retain the rows in memory, causing memory pressure if too many rows are returned.
The Scan query can return all the rows without issuing another pagination query.

In addition to straightforward usage where a Scan query is issued to the Broker, the Scan query can also be issued directly to Historical processes or streaming ingestion tasks. This can be useful if you want to retrieve large amounts of data in parallel.

An example Scan query object is shown below:

 {
   "queryType": "scan",
   "dataSource": "wikipedia",
   "resultFormat": "list",
   "columns":[],
   "intervals": [
     "2013-01-01/2013-01-02"
   ],
   "batchSize":20480,
   "limit":3
 }

The following are the main parameters for Scan queries:

property	description	required?
queryType	This String should always be "scan"; this is the first thing Druid looks at to figure out how to interpret the query	yes
dataSource	A String or Object defining the data source to query, very similar to a table in a relational database. See DataSource for more information.	yes
intervals	A JSON Object representing ISO-8601 Intervals. This defines the time ranges to run the query over.	yes
resultFormat	How the results are represented: list, compactedList or valueVector. Currently only list and compactedList are supported. Default is list	no
filter	See Filters	no
columns	A String array of dimensions and metrics to scan. If left empty, all dimensions and metrics are returned.	no
batchSize	How many rows buffered before return to client. Default is 20480	no
limit	How many rows to return. If not specified, all rows will be returned.	no
order	The ordering of returned rows based on timestamp. "ascending", "descending", and "none" (default) are supported. Currently, "ascending" and "descending" are only supported for queries where the limit is less than druid.query.scan.maxRowsOrderedInMemory. Scan queries that are either legacy mode or have a limit greater than druid.query.scan.maxRowsOrderedInMemory will not be time-ordered and default to a order of "none".	none
legacy	Return results consistent with the legacy "scan-query" contrib extension. Defaults to the value set by druid.query.scan.legacy, which in turn defaults to false. See Legacy mode for details.	no
context	An additional JSON Object which can be used to specify certain flags.	no

Example results

The format of the result when resultFormat equals list:

 [{
    "segmentId" : "wikipedia_editstream_2012-12-29T00:00:00.000Z_2013-01-10T08:00:00.000Z_2013-01-10T08:13:47.830Z_v9",
    "columns" : [
      "timestamp",
      "robot",
      "namespace",
      "anonymous",
      "unpatrolled",
      "page",
      "language",
      "newpage",
      "user",
      "count",
      "added",
      "delta",
      "variation",
      "deleted"
    ],
    "events" : [ {
        "timestamp" : "2013-01-01T00:00:00.000Z",
        "robot" : "1",
        "namespace" : "article",
        "anonymous" : "0",
        "unpatrolled" : "0",
        "page" : "11._korpus_(NOVJ)",
        "language" : "sl",
        "newpage" : "0",
        "user" : "EmausBot",
        "count" : 1.0,
        "added" : 39.0,
        "delta" : 39.0,
        "variation" : 39.0,
        "deleted" : 0.0
    }, {
        "timestamp" : "2013-01-01T00:00:00.000Z",
        "robot" : "0",
        "namespace" : "article",
        "anonymous" : "0",
        "unpatrolled" : "0",
        "page" : "112_U.S._580",
        "language" : "en",
        "newpage" : "1",
        "user" : "MZMcBride",
        "count" : 1.0,
        "added" : 70.0,
        "delta" : 70.0,
        "variation" : 70.0,
        "deleted" : 0.0
    }, {
        "timestamp" : "2013-01-01T00:00:00.000Z",
        "robot" : "0",
        "namespace" : "article",
        "anonymous" : "0",
        "unpatrolled" : "0",
        "page" : "113_U.S._243",
        "language" : "en",
        "newpage" : "1",
        "user" : "MZMcBride",
        "count" : 1.0,
        "added" : 77.0,
        "delta" : 77.0,
        "variation" : 77.0,
        "deleted" : 0.0
    } ]
} ]

The format of the result when resultFormat equals compactedList:

 [{
    "segmentId" : "wikipedia_editstream_2012-12-29T00:00:00.000Z_2013-01-10T08:00:00.000Z_2013-01-10T08:13:47.830Z_v9",
    "columns" : [
      "timestamp", "robot", "namespace", "anonymous", "unpatrolled", "page", "language", "newpage", "user", "count", "added", "delta", "variation", "deleted"
    ],
    "events" : [
     ["2013-01-01T00:00:00.000Z", "1", "article", "0", "0", "11._korpus_(NOVJ)", "sl", "0", "EmausBot", 1.0, 39.0, 39.0, 39.0, 0.0],
     ["2013-01-01T00:00:00.000Z", "0", "article", "0", "0", "112_U.S._580", "en", "1", "MZMcBride", 1.0, 70.0, 70.0, 70.0, 0.0],
     ["2013-01-01T00:00:00.000Z", "0", "article", "0", "0", "113_U.S._243", "en", "1", "MZMcBride", 1.0, 77.0, 77.0, 77.0, 0.0]
    ]
} ]

Time Ordering

The Scan query currently supports ordering based on timestamp for non-legacy queries. Note that using time ordering will yield results that do not indicate which segment rows are from (segmentId will show up as null). Furthermore, time ordering is only supported where the result set limit is less than druid.query.scan.maxRowsQueuedForOrdering rows or fewer than druid.query.scan.maxSegmentsOrderedInMemory segments are scanned per Historical. The reasoning behind these limitations is that the implementation of time ordering uses two strategies that can consume too much heap memory if left unbounded. These strategies (listed below) are chosen on a per-Historical basis depending on query result set limit and the number of segments being scanned.

1. Priority Queue: Each segment on a Historical is opened sequentially. Every row is added to a bounded priority queue which is ordered by timestamp. For every row above the result set limit, the row with the earliest (if descending) or latest (if ascending) timestamp will be dequeued. After every row has been processed, the sorted contents of the priority queue are streamed back to the Broker(s) in batches. Attempting to load too many rows into memory runs the risk of Historical nodes running out of memory. The druid.query.scan.maxRowsQueuedForOrdering property protects from this by limiting the number of rows in the query result set when time ordering is used.

2. N-Way Merge: Each segment on a Historical is opened in parallel. Since each segment's rows are already time-ordered, an n-way merge can be performed on the results from each segment. This approach doesn't persist the entire result set in memory (like the Priority Queue) as it streams back batches as they are returned from the merge function. However, attempting to query too many segments could also result in high memory usage due to the need to open decompression and decoding buffers for each. The druid.query.scan.maxSegmentsOrderedInMemory limit protects from this by capping the number of segments opened per historical when time ordering is used.

Both druid.query.scan.maxRowsQueuedForOrdering and druid.query.scan.maxSegmentsOrderedInMemory are configurable and can be tuned based on hardware specs and number of dimensions being queried.

Legacy mode

The Scan query supports a legacy mode designed for protocol compatibility with the former scan-query contrib extension. In legacy mode you can expect the following behavior changes:

• The __time column is returned as "timestamp" rather than "__time". This will take precedence over any other column you may have that is named "timestamp".
• The __time column is included in the list of columns even if you do not specifically ask for it.
• Timestamps are returned as ISO8601 time strings rather than integers (milliseconds since 1970-01-01 00:00:00 UTC).

Legacy mode can be triggered either by passing "legacy" : true in your query JSON, or by setting druid.query.scan.legacy = true on your Druid processes. If you were previously using the scan-query contrib extension, the best way to migrate is to activate legacy mode during a rolling upgrade, then switch it off after the upgrade is complete.

Configuration Properties

property	description	values	default
druid.query.scan.maxRowsQueuedForOrdering	The maximum number of rows returned when time ordering is used	An integer in [0, 2147483647]	100000
druid.query.scan.maxSegmentsOrderedInMemory	The maximum number of segments scanned per historical when time ordering is used	An integer in [0, 2147483647]	50
druid.query.scan.legacy	Whether legacy mode should be turned on for Scan queries	true or false	false